The present invention relates to a light-emitting apparatus having a substrate on which a light-emitting device is formed. The light-emitting device is an organic electroluminescent device (organic EL device) or an inorganic electroluminescent element (inorganic EL elements). Such a device has a light-emitting layer held between a pair of electrodes. The present invention also pertains to a method for forming the apparatus.
Lighting systems and displays using a light-emitting apparatus such as an organic electroluminescent apparatus (organic EL apparatus) and an inorganic electroluminescent apparatus (inorganic EL apparatus) have been proposed.
Such light-emitting apparatuses are known for low extraction efficiency. That is, the ratio of the light that is extracted outside from the apparatus to the light generated by a light-emitting layer is low. This is because layers forming a light-emitting apparatus have different refractive indices.
For example, in an organic EL apparatus of a bottom emission type shown in FIG. 22, all the light that is generated by an organic light-emitting layer 211 is extracted outside from the apparatus through a transparent first electrode 201 and a transparent substrate 100. Some of the light attenuates in the apparatus as a ray h4, and some of the light is extracted outside the apparatus through an edge of the apparatus as a ray h5.
In this manner, the conventional light-emitting apparatus cannot extract all the light generated by the light-emitting layer 211 to the outside of the apparatus from a light exit surface A.
With this respect, a first prior art has been proposed in which an uneven surface having asperities is formed on a transparent substrate, and an organic EL device is formed on the uneven surface (for example, Japanese Laid-Open Patent Publication No. 1-186587). The first prior art discloses a technique related to a display using an inorganic EL apparatus, in which one pixel is provided for each of pits and projections of the asperities so that light trapped in a light-emitting film is reflected by steps formed by the asperities. Accordingly, the amount of light extracted outside the transparent substrate is increased.
The extraction efficiency of a light-emitting apparatus can be improved by reflecting light that is directed in a direction opposite a light extraction side with respect to a light-emitting layer. Specifically, an electrode provided on a side of the light-emitting layer that is opposite from the light extraction side may be made of a light reflecting material such as Al. Alternatively, a light reflecting member may be provided on a side of the light-emitting layer that is opposite from the light extraction side. In these cases, however, when the light-emitting layer is not producing light, specular reflection of external light occurs.
With this respect, a second prior art has been proposed in which dot-like asperities are formed at random on a transparent substrate, and electrodes and a light-emitting layer are provided on the asperities (for example, Japanese Laid-Open Patent Publication No 2000-40584). The second prior art prevents glare due to specular reflection of the metal electrode.
A third prior art discloses a technique applied to a substrate of an organic EL apparatus that has organic EL device and is used as a backlight of a liquid crystal display panel. The technique pertains to forming asperities on the substrate of the apparatus (for example, Japanese Laid-Open Patent Publication No. 9-50031). According to the third prior art, the substrate with the asperities functions as a diffusion plate, so that, when the organic EL apparatus is used as a reflection plate, specular reflection is prevented.
However, as in the first prior art, forming an uneven surface having asperities on the substrate and forming light-emitting device on the uneven surface does not necessarily improve the extraction efficiency. Depending on the shapes of the asperities, the amount of light extracted from the light exit surface is decreased compared to a case of a flat light-emitting apparatus on which no uneven surface is formed.
Also, depending on the shapes of the asperities, the following problems occur. That is, when forming light-emitting device on the uneven surface as shown in FIG. 23(a), the thickness of each of the layers forming the light-emitting device (particularly, a light-emitting layer) varies according to locations. Also, as shown in FIG. 23(b), electrodes 201, 221 could contact each other (for example, at a location E).
As shown in FIG. 23(a), a portion of a light-emitting layer 211 (for example, a portion C in the drawing) that is thinner than other portions (for example, a portion B) has a lower resistance and thus the current passes easily. This results in a higher brightness at the portion C compared to other portions (the portion B). A great amount of current at the thin portion C increases the temperature at the portion C. This further reduces the resistance at the portion C. Accordingly, a greater current passes through the portion C, and the brightness is further increased. Accordingly, the apparatus might have an uneven brightness. Further, the phenomenon causes a significant amount of current to pass through specific portions of the light-emitting layer 211, and thus shortens the lifetime of these portions. As a result, the lifetime of the apparatus is shortened.
If a portion where no light-emitting layer 211 exists (for example, a portion E in the drawing) as shown in FIG. 23(b), a significant amount of current passes through such a portion. Thus, the amount of current that passes through the light-emitting layer 211 is decreased, for example, in a portion D. As a result, sufficient brightness cannot be obtained.
Further, a light-emitting apparatus is required to have a high brightness in a specific direction. This is because a user using or viewing a light-emitting apparatus is typically views the apparatus from a particular direction in relation to the apparatus (generally, in a normal direction of a light extraction side, or a light exit surface, of the apparatus).
On the other hand, when a light-emitting apparatus is used as a reflection plate as in the third prior art, the brightness of reflected light in a specific direction needs to be increased in addition to preventing specular reflection. FIG. 24 illustrates a case where a light-emitting apparatus is used as a backlight of a liquid crystal display unit 8 under a light source 9, which is, for example, the sun or a fluorescent tube. In this case, when light from the light source 9 reaches the display unit 8 along a direction shifted from the normal H to a display surface 80 by an angle θ (the angle θ is approximately 20° to 40°, and preferably, is approximately 30°), not only specular reflection needs to be prevented, but also, the brightness in the direction of the normal H to the display surface 80, or in a direction where the angle θ is approximately 0°, needs to be increased.